2000 BMW 316 UNKNOWN NOISE

ENGINE18

2000 BMW 316

Noises problem
2000 BMW 316 4 cyl Two Wheel Drive Manual 85000 miles

Hi- my BMW 316i SE has developed a high pitched squeal from under centre of the car. It starts after 15-20 minutes of driving and stays until the car/engine cools. It remains on when stationary and in or out of gear. It does stop momentarily when applying acceleration at pull aways then returns. It is not belts or air leaks and mechanics? Say it is from around the drive shaft area.? Help.